Cleaning operations are becoming more and more of a burden on industry because of the ever-stricter requirements, being made on the environmental disposition of compounds used in the cleaning operations and resulting effluents. Cleaning operations affected include those involving clothing, rugs and furnishings, as well as those of a more industrial nature such as involving the cleaning and degreasing of metals, ceramics, plastics and other materials. Solvent cleaning processes, those using a solvent to degrease and clean, are the most prevalent. There are two types of solvent cleaning processes: open and closed. Open systems are still the most commonly used, but their appeal is shrinking with increasing demands of environmental safety. Open systems include such approaches as solvent vapor degreasing, solvent ultrasonic cleaning, cold or hot solvent dipped and solvent spray systems. These systems suffer from a number of shortcomings, among the most important of which are the contamination of the environment and the cost of constantly replenishing the non-recoverable solvent. In addition, the cost of equipment to contain the vapor and to properly dispose of the vapor and liquid waste is becoming more and more formidable.
The closed systems, so-called, attempt to combat these problems but with indifferent success. The loss of solvent as vapor and liquid still occurs even in a so-called closed system because the vapor escapes when the cleaned parts are removed and the parts carry off solvent which clings to their surfaces and resides in the pores of the material. Further, attempts to recover the solvent are expensive and less than totally successful. For even though the system is "closed" when the solvent is introduced to the closed chamber, it mixes with the air there. After the cleaning operation the solvent in liquid form can be easily separated from the air, but not so with solvent in vapor form. That requires a major effort. Even if the air and solvent are condensed, only a small amount of solvent can be recovered. Incineration is one technique for getting rid of solvent but that requires significant investment in special equipment, extra heat energy and it loses the solvent, which must then be replenished. Steam stripping is a technique which actually recovers the solvent, but it too requires special equipment and heat energy to make the steam. In addition, the steam must be condensed to water and then separated from the solvent.
Thus, conventional cleaning systems have problems in the area of hazardous emissions and solvent recovery. They are generally limited to operating at specific temperatures and pressures. They typically do not dry all the solvent off the objects before exposing them to the atmosphere. They utilize heat energy during a substantial part, if not all, of the cleaning cycle. Conventional systems also need a great deal of solvent to fill their cleaning tanks and require additional energy input to pump the solvent through the system. In addition, standard solvent vapor cleaning systems must use solvents whose vapors are heavier than air. These vapors are confined in a blanket over the boiling solvent by using expensive refrigerator coils and by limiting the dimensions of the system tank. These systems operate at fixed temperatures which are determined by the boiling point of the solvent at atmospheric pressure.
A different approach, known as vacuum degreasing, avoids some of the problems of solvent cleaning. In this approach the contaminants are exposed to a high temperature, low-pressure environment in order to reach pressures below the vapor pressure of the contaminant. Essentially the contaminant is boiled off the parts. The problem here, however, is that the contaminants generally have a very low vapor pressure. Consequently, extremely high vacuums and/or temperatures are required. Although solvents are not emitted, the contaminant often is emitted which itself can become a problem. The process is generally costly due to the sub torr pressures and high temperatures required. In many cases non-volatile residue, either present in the contaminant originally (i.e., sulfur residue) or resulting from a breakdown of the contaminant due to the high temperature requirements (carbon deposits) are often left behind on the parts. The pollution abatement energy costs and cleaning efficiency requirements strongly limit the applications of such a system.